Recently, downsizing has been required for display units used for computers. With the widespread use of personal computers, in particular, a great deal of attention has been paid to flat displays, mainly liquid crystal displays. However, no flat display units have been developed, which can cope with electron tubes such as cathode ray tubes in terms of size, resolution, and cost. For this reason, electron tubes such as cathode ray tubes are in urgent need of reducing their total lengths and weights.
Similar needs are increasing with respect to traveling wave tubes designed to be mounted in satellites. With such needs, demands have arisen for compact, low-profile, lightweight electron guns including cathode assemblies as tube parts.
High-speed operations are often required for high-output traveling wave tubes. A general tube uses a hot cathode assembly as an electron source, and hence the temperature rise time in the cathode assembly dominates the time required for the stable operation of the tube. That is, quick heating of the cathode assembly is required for the quick operation of the tube.
Attempts have been made to develop low-profile, lightweight display units using electron tubes. For example, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-58970, a low-profile display unit using an electron tube in which a plurality of electron guns are arranged has been proposed.
A short, low-power-consumption, fast electron gun is required as each electron gun mounted in the electron tube of such a display unit to reduce the profile and weight of the display unit and improve its performance.
A conventional electron tube will be described below with reference to FIG. 67. FIG. 67 is a sectional view showing portions around the cathode assembly in the electron gun assembly used in the conventional electron tube.
The cathode assembly includes a cathode sleeve 1 consisting of an alloy such as nichrome. A base metal layer 2 consisting of nickel doped with a small amount of reducing material is fixed to one end of the cathode sleeve 1. The surface of the base metal layer 2 is coated with an emissive material 3 consisting of barium oxide (BaO), strontium oxide (SrO), calcium oxide (CaO), or the like. A cathode base member 4 is constituted by the base metal layer 2 and the emissive material 3. As the cathode base member 4, in addition to the above structure, a so-called impregnated cathode base member obtained by impregnating a porous cathode base member with an emissive material such as barium oxide (BaO), strontium oxide (SrO), or aluminum oxide (Al.sub.2 O.sub.3) is used.
The cathode sleeve 1 is fixed to a cathode holder 6 consisting of Kovar (Fe--Ni--Co-based alloy) through a strap 5 consisting of invar (Fe--Ni-based alloy) as a low-thermal-expansion alloy. The cathode holder 6 =-surrounds the cathode sleeve 1 through a reflector 7 consisting of an Ni-based refractory alloy material for blocking/reflecting heat from the cathode sleeve 1. The cathode holder 6 is fixed to a cathode support strap 9 consisting of a stainless-steel-based alloy through a cathode support cylinder 8 consisting of a stainless-steel-based alloy.
A heater 10 for heating the cathode is mounted in the cathode sleeve 1. The heater 10 is obtained by helically winding an Re--W alloy wire, and coating its surface with aluminum oxide (Al.sub.2 O.sub.3) as an insulating material. The heater 10 is an elongated member extending along the longitudinal direction of the electron gun. The heater 10 is inserted into the cathode sleeve 1 through the other end thereof such that the end portions of the heater protrude from the cathode sleeve 1. The end portions of the heater 10 are fixed to a heater tab strap 12 consisting of a stainless-steel-based alloy through a heater tab 11 consisting of a stainless-steel-based alloy. The cathode assembly is constituted by the cathode base member 4 and the above parts.
A first grid 13 consisting of a stainless-steel-based alloy and serving to control an electron flow is placed to oppose the cathode base member 4. The cathode assembly, the first grid 13, and the like constitute an electron gun assembly 15. A bead glass 14 surrounds this electron gun assembly 15. The cathode support strap 9, the heater tab strap 12, and the first grid 13 are fixed to the bead glass 14.
As the cathode base member, a member using an impregnated cathode obtained by impregnating a base metal layer with an emissive material is provided instead of a member using the above oxide-coated cathode. A thin iridium layer may be formed on the electron emission surface of the cathode base member.
For example, the following dimensions are set for the electron gun assembly having the above structure. The cathode sleeve 1 is 4 mm long. The base metal layer 2 is 1.1 mm long. The length from the surface of the emissive material 3 to the lower end of the cathode holder 6 is 9.0 mm. The distance from the upper end of the first grid 13 to the surface of the emissive material 3 is 0.5 mm. The distance from the lower end of the cathode holder 6 to the lower end of the heater tab 11 is 5 mm. The total length of the conventional electron gun assembly is therefore 14.5 mm.
In the general cathode assembly, as the heater 10, a refractory metal wire coiled into a cylindrical shape, a helical shape, or the like is used. For example, a tungsten wire having a diameter of about 50 .mu.m is used as the heater of a cathode assembly for a cathode ray tube. Such a wire needs to have a length of about 100 to 130 mm to heat the cathode to the rated temperature. When this wire is formed into a heater with insulation being maintained, the heater has a diameter of about 1.0 mm and a total length of about 7 mm. This length is 90% or more of the total length of the cathode assembly. That is, the heater must be reduced in size and profile to reduce the size and profile of the cathode. However, the existing heaters used in the conventional cathode assemblies have reached their limits in terms of dimensions.
In the above cathode assembly, the cathode base member 4 is the so-called oxide cathode, whose operating temperature is 830.degree. C. The heater power required to raise the cathode temperature to this operating temperature is 0.35 W. In addition, it takes 10 seconds for the cathode assembly to stabilize displayed images after the power is turned on.
The fast operation characteristics, i.e., fast heating, of the cathode assembly are dominated by heat conduction from the heater to the cathode base member. It is ideal that heat from the heater be directly transmitted to only the cathode assembly.
The cathode base member in the cathode assembly is heated through two heat transmission routes. One route is the route through which the cathode is directly heated by radiant heat from the heater. The other route is the route through which the cathode base member is heated by heat diffusion in the assembly which is caused when the support cylinder is heated by radiant heat from the heater. The time required to set the cathode base member in a stable, high-temperature state is dominated by heat conduction through the latter route. This causes a decrease in temperature rise rate.
In the cathode assembly having the above structure, however, heat conduction to the sleeve cannot be prevented. As a method of quickly heating the cathode, a method of decreasing the mass of the cathode base member or the sleeve is used. In this case, however, problems are posed in terms of thermal distortion of the cathode itself; limitations are imposed on the application of this method. In an existing traveling wave tube, it takes three minutes or more for the cathode base member to reach a brightness temperature of 900 to 1,050.degree. C.b and stabilize the tube operation after the heater power is turned on.
The following problems are posed when such a conventional electron tube is used for a low-profile display unit.
The total length of the electron gun assembly is too long. An electron tube used for a low-profile display unit is required to have a total length of 130 mm or less. The length from the first grid to the lower end of the heater tab in the conventional electron gun assembly, i.e., 14.5 mm, is too long to meet this requirement.
A plurality of electron gun assemblies are used for the electron tube used for the above low-profile display unit. For example, 24 electron gun assemblies are used for a 40-inch tube. For the overall electron tube, total heater power corresponding to the heater power required for one electron gun assembly (cathode assembly).times.the number of electron gun assemblies is required. For this reason, the total heater power required for the overall electron tube must be minimized.
The heater power required for the cathode assembly in the conventional electron gun assembly is not sufficiently low. If a plurality of conventional electron gun assemblies are used, the total heater power required for the overall electron tube becomes high. If, for example, conventional electron gun assemblies are used, the total heater power becomes 0.35 W.times.24 (assemblies)=8.4 W, posing a problem in terms of power saving in the electron tube.
In addition, in an electron tube having a plurality of electron gun assemblies, if the fast operation characteristics of the cathode assemblies of the respective electron gun assemblies vary, the overall image displayed on the display unit after the power is turned on is disturbed. In order to prevent this image disturbance, therefore, the fast operation characteristics of each electron gun assembly must be improved.
In a conventional electron gun assembly, however, it takes 10 seconds to obtain a stable image. This rise time is too long to regard the fast operation characteristics as good characteristics.
As described above, according to the cathode assembly in the conventional electron tube, it is difficult to attain the decreases in size and power consumption, and the fast operation characteristics. Demands have therefore arisen for a cathode assembly having a new structure. A cathode assembly which can solve such a problem is disclosed in U.S. Pat. No. 5,015,908.
The heater unit used in the cathode assembly disclosed in this reference is obtained by forming a heating member having an anisotropic pyrolytic graphite (APG) heater pattern on a substrate consisting of anisotropic pyrolytic boron nitride (APBN). This unit is very thin; about 1 mm thick. In addition, the heater unit allows the lower surface of an insulating substrate to be directly connected to the cathode assembly. That is, the fast operation characteristics can be attained with decreases in size, profile, and thermal capacity.
The above cathode assembly is suitable for an electron tube having a large structure such as a crystron or a traveling wave tube. However, no special consideration is given to a compact, low-power-consumption electron tube which is mass-produced, e.g., a cathode ray tube.
In the conventional cathode assembly, there is a large difference in thermal expansion coefficient between the cathode assembly and the heater or the heater substrate, resulting in poor joining properties. For this reason, the cathode assembly is joined to the insulating substrate through a thin tungsten layer and tungsten and nickel powders by sintering, resulting in a very complicated manufacturing process. Problems are therefore posed in the conventional cathode assembly in terms of mass production and manufacturing cost.
That is, the heater unit and the heating member are fixed by coating the outermost surface of the insulating substrate with tungsten, inserting nickel and tungsten powders between the outermost surface, the cathode lower surface, and the sleeve, and sintering the resultant structure at 1,300.degree. C. When these members are joined by sintering, however, the joining strength is very low. During the operation of the cathode assembly, therefore, the joined members may peel off. In addition, as sintering progresses with the operation of the cathode assembly, the heater characteristics very likely change.
A problem is also left unsolved in forming an electrode from the heater unit. The electrode of the heater is mechanically joined to the heating member by a mechanical joint by screwing or pressing. For this reason, a connection failure may be caused by thermal expansion upon heating. In addition, in a compact cathode base member having a diameter of about 1 mm, e.g., a cathode base member used in a cathode ray tube, the heater power increases owing to the thermal capacity of the screwed portion.
In a color cathode ray tube, three cathode assemblies are used per electron gun assembly, and the cathode assemblies are fixed while the spaces between the first grid and the respective cathode assemblies are measured by an air micrometer or the like to make the distances constant. In this case, if the positions where the cathode assemblies are fixed vary, electrons emitted from the respective electron gun assemblies vary when the switch of the cathode ray tube is turned on (the power switch of the electron tube is turned on), resulting in imperfect color reproduction. Therefore, the spaces between the first grid and the cathode assemblies must be set with high precision.